Insulating oil compositions



United States PatentQ 2,921,011 INSULATING'OIL COMPOSITIONS Hermann Ordelt, Hamburg-Neugraben, and Eva-Maria Jahn, Hamburg-Harburg, Germany, assignors to The British Petroleum Company Limited, London, England, a British joint-stock corporation No Drawing. Application April 27, 1956 Serial No. 581,006

Claims priority, application Germany April 29, 1955 5 Claims. (Cl. 208-14) This invention relates to improved insulating oil compositions.

Insulating oils are used in a number of applications, such as in transformers, condensers and cables. Such oils must, of course, have a high resistance and should also have a low dielectric Watt loss. This latter property may be characterised by the value of the loss angle, which is the difference between 90 and the angle of lead of current over voltage; it may be also characterised by the loss factor, which is the watt loss divided by the effective product of volts and amps. It is important that these properties be maintained under conditions of use, and the oils must therefore also possess resistance to the phenomena of ageing and gassing. Ageing comprises oxidation due to aerial oxygen, and polymerisation and condensation. These effects lead to the formation of acid products and sludge. Gassing is an effect which is liable to occur in oils under the influence of a hightension alternating electric field: hydrogen may be split off in the form of small bubbles and small bubbles of air may increase in size. Ageing and gassing may result in the corrosion of metal and other parts and in the reduction of insulation and dielectric strength. A good insulating oil should not be subject to ageing and should be capable of absorbing gas.

Mineral oils which are used as insulating oils are usually low viscosity distillates. They must be highly refined in order to obtain good ageing resistance, but it is often found that they are then not capable of absorbing gas. It is difficult to achieve a degree of refinement which will provide both good ageing resistance and good gas absorption; also, to achieve as low a loss angle as possible it has been found that oils must be highly refined, which is again not compatible with good gas absorption.

I It has been suggested that refined solvent extract from spindle oil be added to a refined oil in an attempt to overcome this difficulty. It is usually necessary, however, to add a comparatively large amount of such extract to the base oil and, although good gas absorption properties may be achieved, the ageing resistance and loss angle may be even less satisfactory than those possessed by the base oil. A further point is that the viscosity of the composition may be increased to an undesirable degree.

The principal object of the present invention is to provide an insulating oil composition with good resistance to ageing and gassing and with a low loss angle.

According to the present invention therefore, an insulating oil composition consists essentially of a mixture of a mineral oil and 0.1-l0% by weight of a hydrocarbon fraction, the said fraction being substantially free from olefins and containing at least 20%, and preferably at least 50% by weight of monocyclic aromatics and having a distillation point of at least 160 C. and a 90% distillation point of not more than 300 C.

It has been found possible by this means to prepare insulating oils possessing all the required properties enu- "ice merated above and with none of the hitherto attendant disadvantages. 'It has'also been found possible by this means to achieve the desired results by the addition of a smaller amount of the said fraction than was necessary with the extracts previously used. A yet further advantage. which has been discovered is that it has proved possible to refine the base oil and the added material together after blending.

The hydrocarbon fraction may advantageously be derived from a catalytic reforming process, especially the catalytic reforming process. of the Universal Oil Products Company using a solid cataylst containing amember of the platinum group. Platformer bottoms, that is, bottoms produced by the Universal Oil Products Companys catalytic reforming process aforesaid, are particularly suitable.

Materials other than platformer bottoms which have been used to good effect include those obtained by the solvent extraction of alkylated, hydrogenated polymerization gasoline, hydrogenated cat cracker gas oil and a cracking hydrogenation product. In the case of the extracts from the hydrogenated polymerization gasoline and the hydrogenated cat cracker gas oil it was necessary to refine them. The other two materials may be refined if necessary. The original polymerization gasoline and cat cracker gas oil had to be hydrogenated as they contained olefins, which rendered them unsuitable.

The following examples illustrate the preparation of insulating oil compositions according to the invention:

Example 1 A transformer oil was tested for gas resistance according to the Erdol and Kohle, 3rd year, pp. 427-437, 1950, Th. Worner, Nurnberg. 20 cc. of the oil were exposed at C. to an electric field of 8.5 kv. and 50 Hertz and the oil was found to split off gas. The ageing value of this oil according to Baader (see Mineralole und verwandte Produkte, Carl Zerbe, 1952, pp. 945-6) was (with copper as contact), a saponification number of 0.1 mg. KOH per cc. of oil.

To this was added 1.5% of a platformer gasoline fraction, in the boiling range -230 C. The gas absorption increased to 20 cc. absorption after 24 hours. The Baader saponification number dropped from 0.1 to 0.06 and the loss factor from 0.3.10- to below 0.1.10-

Example 2 To a transformer oil of gas releasing quality was added 1% by weight of platformer bottoms. The gas absorption factor obtained was 0.32. The Baader saponification number dropped from 0.53 to 0.50.

Example 3 To the transformer oil mentioned in Example 2 was added 2% by weight of platformer bottoms. The gas absorption factor obtained was 0.61. The Baader saponification number dropped from 0.53 to 0.35.

Example' 4 To the transformer oil mentioned in Example 2 was added 3% by weight of platformer bottoms. The gas absorption factor obtained was 0.90. The Baader saponification number dropped from 0.53 to 0.32.

Example 5 We claim:

1. An insulating oil composition consisting essentially of a mixture of a mineral ioil and 01-10% by weight of a multi-constituent hydrocarbon fraction boiling over a range of temperatures between an initial boiling point and a final boiling point, the said fraction being a platformer bottoms substantially free from olefins and containing, in addition to its other constituents, at least 20% by weight of monocyclic aromatics, and having a 10% distillation point of at least 160 ,C. and a 90% distillation point of not more than 300 C.

2. An insulating oil composition according to claim 1, in which the hydrocarbon fraction contains at least 50% by weight of monocyclic aromatics.

3. An insulating oil composition according to claim I,

in which at least 95% by weight of the aromatic hydrocarbons present in the composition are'monocyclic.

4. An insulating oil composition according to claim 1, containing 2-4% by weight of the hydrocarbon fraction. 5. An insulating oil composition according to claim 1, in which the hydrocarbon fraction is a solvent extract of a platformer bottoms.

References Cited in the file of this patent UNITED STATES PATENTS Hyde June 5, 1945 

1. AN INSULATING OIL COMPOSITION CONSISTING ESSENTIALLY OF A MIXTURE OF A MINERAL OIL AND 0.1-10% BY WEIGHT OF A MULTI-CONSTITUENT HYDROCARBON FRACTION BOILING OVER A RANGE OF TEMPERATURES BETWEEN AN INITIAL BOILING POINT AND A FINAL BOILING POINT, THE SAID FRACTION BEING A PLATFORMER BOTTOMS SUBSTANTIALLY FREE FROM OLEFINS AND CONTAINING, IN ADDITION TO ITS OTHER CONSTITUENTS, AT LEAST 20% BY WEIGHT OF MONOCYCLIC AROMATICS, AND HAVING A 10% DISTILLATION POINT OF AT LEAST 160*C., AND A 90% DISTILLATION POINT OF NOT MORE THAN 300*C. 